Method and apparatus for beam steering in a wireless communications system

ABSTRACT

A method and apparatus is provided that allows M transceivers to transmit/receive using M2 N  distinct beams using passive beam steering. This provides for the use of arbitrary narrow beams with a number of transceivers that is a fraction of the number of beams but ensures 360° coverage. In other words it permits significant improvements in the link budget with a minimal rise in the cost of the BS. The apparatus includes M distribution switches applied to 2 N  passive beam forming networks each coupled to M antennas. The method and apparatus are compatible with TDM in the downlink and TDMA in the uplink.

FIELD OF THE INVENTION

The present invention relates to wireless communications systems and isparticularly concerned with beam steering.

BACKGROUND OF THE INVENTION

An essential part of any wireless link is the design of the antenna andthe choice of its beam width (or angle) and its gain. In generalantennas with narrower beam provide higher gains.

The gain of the antenna contributes twice in the link budget: both attransmission and at reception. At transmission, the effective incidentradiated power (EIRP) [dBm] is the sum between the antenna gain G_(T)[dBi] and the transmitter power P [dBm].

-   -   EIRP[dBm]=P[dBm]+G_(T)[dBi]

At reception, the signal level S[dBm] at input of the receiver is thesum between the antenna gain G_(R) and the transmitted EIRP minus thepath loss PL [dBi].

-   -   S[dBm]=G_(R)[dBi]+EIRP[dBm]−PL[dBi]

The link budget and consequently the coverage can be improved by raisingthe transmitter power P or by raising the antenna gains G_(T) or G_(R).For a transceiver that use the same antenna to transmit and receive,i.e. G_(T)=G_(R), increasing the antenna gain has positive effects onboth transmission and reception while increasing the power improves onlythe transmission. For symmetric links (all participant systems have thesame P and G_(T)=G_(R)), increasing the antenna gain has double effectthan increasing the transmitter power P.

The EIRP in each frequency band is usual limited by regulatory bodieslike Federal Communications Commission in USA. In such cases, the onlyway to improve the link budget and the coverage is to raise the gain ofthe antenna at the receiver G_(R).

When EIRP is limited, rising the antenna gain at the transmitter G_(T)has to be associated with a corresponding reduction in the power of thetransmitter P and implicitly a reduction in the cost of the poweramplifier (PA).

Antennas with narrower beams provide more spatial selectivity, which inturn, improves the system immunity to interference.

With current technologies, the advantages of using high-gain,narrow-beam antennas are offset in the design of a base-station (BS) bythe price of the transceivers needed to obtain 360° coverage. Forexample, a 23 dBi pencil-beam (same beam width in the vertical andhorizontal plane) antenna will have a beam with of only 14°. Thus, inorder to ensure 360° coverage with current technologies, we would need26 antennas and consequently 26 transceivers.

It is known in wireless systems to use beam forming to emulate a highgain antenna using multiple low-gain antennas. This is achieved using asystem as depicted in FIG. 1. A wireless system 10 includes atransceiver 12 coupled to a phase-delay passive network 14 coupled to aplurality of antennas 16 as in the system of FIG. 1. A phase-delaynetwork is inserted between the transceiver and the antennas.

In operation, at transmission, the phase-delay network 10 distributesthe signal from the transceiver 12 to all antennas 16. At reception thenetwork combines the signal received from all antennas 16 and passes theresulting signal to the transceiver 12. The phase and delay for eachantenna are established in accordance with the position of the antennassuch that the desired beam width and direction are obtained.

An extension of the passive beam forming uses several transceivers 12with multiple-input phase-delay network. It has been shown that such anetwork can be implemented and produces beams With gain higher than ofthe constituent antennas if:

-   -   1. The number of transceivers does not exceed the number of        antennas.    -   2. The transceivers operate on close but different frequencies        to avoid cross-talk between beams.

Referring to FIG. 2, there is illustrated a known wireless system foractive beam steering. The wireless system 20 includes a transceivercommon part 22 coupled to an electronically controlled phase delayactive network 24 coupled to a plurality of transceiver RF parts 26 eachcoupled to a corresponding one of a plurality of antennas 28.

Active beam steering is another extension of beam forming, in which thephase-delay network is electronically controlled. By trimming phases anddelays, the resulting beam can be steered into the desired direction.

Both known beam forming of FIG. 1 and steering of FIG. 2 require preciseamplitude, phase and delay control in the phase-delay network. They alsorequire precise alignment of the antennas and precise amplitude, phaseand delay matching between RF cables. In practical systems, theprecision of these elements is the most important factor that limits theachievable antenna gain. Precision is especially hard to maintain withbeam steering where phase and delay parameters are variable. Practicalimplementations of beam steering use phase-delay networks implemented inbase-band processors to ensure precise delay and phase control.Therefore in active beam steering systems the RF part of the transceiveris replicated for each antenna, as shown in FIG. 2.

Active beam steering systems are very expensive because they requirereplication of the RF subunit for each antenna when multiple antennasare used to achieve a single beam.

Even with the phase-delay network implemented in base-band, the activebeam-steering systems require precise amplitude, phase and delaymatching between RF subunits. In practice, errors occur and thisseriously limits the maximum achievable antenna gain.

A further concern is that the active beam steering system of FIG. 2offers no upgrade path, an important feature in wireless systemdeployment. In order to add a second beam, one must add an entire newsystem with multiple RF subunits and multiple antennas in addition tothe new transceiver. This could be an important limitation duringwireless system deployment.

Active beam steering may not be compatible with current standards forwireless broadband access. In FIG. 3, an example of an air interface fora wireless system illustrated in a functional block diagram. The airinterface 30 includes a downlink portion 32 and an uplink portion 34.The downlink portion begins with a broadcast segment 36 followed by aplurality of unicast segments 38. The uplink portion 34 includes acontention window 40 and a plurality of scheduled uplinks 42.

As shown in FIG. 3, these standards, e.g. IEEE802.16, employ downlinkbroadcast messages that must be sent from the base-station (BS) to allsubscriber stations (SS) at the same time. They also employ uplinkcontention windows during which BS has to “listen” for new SSs withoutknowing the direction in which it must steer the beam. In order tosupport these features, the beam must be made 360° wide during theseperiods. This may not be acceptable or even possible because, forexample, enlarging the beam from 22° to 360° causes a reduction of theantenna gain of at least 12 dB.

SUMMARY OF THE INVENTION

Accordingly the present invention provides a method and apparatus thatallows M transceivers to transmit/receive using M2^(N) distinct beamsusing passive beam steering.

Advantages of the present invention allows use of arbitrary narrow beamswith a number of transceivers that is a fraction of the number of beamsbut ensures 360° coverage. In other words it permits significantimprovements in the link budget with a minimal rise in the cost of theBS.

Advantages of the present invention entails a method which does notrequire precise positioning of the antennas and does not requireamplitude, phase or delay matching in the RF cabling.

Advantages of the present invention entails a method that requiresreplication of only a small part of the RF stages but it does notrequire amplitude, phase or delay matching between them.

Advantages of the present invention entails a method and apparatus whichallow easy, hot upgrade from M to M+1, M+2 and so on up to M2^(N)transceivers.

Advantages of the present invention entail a method and apparatus whichallow hot downgrade from any number of transceivers grater than M+1 downto M transceivers. It is shown that downgrade paths can be used toprovide a fail-safe system.

Advantages of the present invention include both the upgrades and thedowngrades are performed without affecting the antenna or the beam gainas seen by each subscriber station. In other words upgrades anddowngrades are performed without affecting the RF link budget.

Advantages of the present invention entail a method and apparatus whichare described as applied at RF but it can also be seamlessly applied atIF or base-band. However the cost of the system is minimized wheninvention is applied at RF.

Advantages of the present invention entail a method as shown to becompatible with existing wireless broadband access standards. It isshown that it supports broadcast messages in the downlink and contentionwindows in the uplink without changing the antenna gain and the linkbudget.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features of the invention will become more apparent fromthe following description in which reference is made to the appendeddrawings.

FIG. 1 illustrates a known wireless system with passive beam forming;

FIG. 2 illustrates a known wireless system with active beam steering;

FIG. 3 illustrates in a block diagram an air interface for a wirelesscommunications system;

FIG. 4. illustrates a wireless system in accordance with an embodimentof the present invention;

FIGS. 5 a and 5 b illustrate examples of grouping for M2^(N)=16 for thesystem of FIG. 4;

FIG. 6 illustrates in further detail a 4-way distribution switch for thesystem of FIG. 4;

FIG. 7 illustrates all useful configurations that can be obtained withthe 4-way distribution switch of FIG. 6;

FIG. 8 there is illustrated an 8-way distribution switch for the systemof FIG. 4;

FIG. 9 illustrates upgrade-downgrade paths with the 8-way distributionswitch of FIG. 8;

FIG. 10 illustrates a possible implementation of the cross-switch ofFIGS. 6 and 8;

FIG. 11 illustrates a possible implementation of the straight-switchFIGS. 6 and 8;

FIG. 12 illustrates in a block diagram a protocol for one MAC frame forTDM/TDMA access to 2^(n) beams;

FIG. 13, there is illustrated in a flow chart a beam selection inaccordance with an embodiment of the present invention; and

FIG. 14 illustrates in a block diagram an alternative protocol for oneMAC frame for TDM/TDMA access to 2^(n) beams.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIG. 4. there is illustrated a wireless system inaccordance with an embodiment of the present invention. The wirelesssystem 50 includes a plurality of transceivers 52 a-m coupled to acorresponding plurality of distribution switches 54 a-m. Distributionswitches 54 a-m each having 2^(n) outputs for coupling to correspondinginputs of 2^(n) passive beam forming networks 56 each passivebeam-forming network 56 is connected to a plurality M of antennas 58.

The system of FIG. 4 uses M2^(N) high-gain antennas 58 that are firstgrouped in 2^(N) groups of M antennas each. Each group of M antennas isprocessed by one beam-forming network 56 to form M high-gain beams.Note, that an embodiment of the invention may be applied without thebeam-forming network, in which the beam width and gain are equal to theantenna angle and gain. However, in most cases when a large number ofantennas are used the beam-forming network will be used to reducesignificantly the cost of the antenna system.

In operation, the resulting M2^(N) beams operate on M differentfrequencies to ensure proper operation of the beam-forming network.

Each group of 2^(N) beams operating on the same frequency is processedthrough a distribution switch 54 that allows 1, 2, 3, and up to 2^(N)transceivers 52 to control the 2^(N) beams.

The present passive beam steering permits a top-down approach to thedesign of an upgradeable BS. The designer chooses the beam angle (width)BA based on the performance of the beam forming technology and theantenna availability. The designer chooses also the minimum separationangle SA between active beams operating at the same frequency and theminimum overlapping angle OA between adjacent beams. Then, 360°/(BA−OA)gives the minimum number of sectors needed in the system and360°/(BA+SA) gives the maximum frequency reuse in the system. Thedesigner chooses M and N such that:

-   -   M2^(N)≧360°/(BA−OA) and 2^(N)≦360°/(BA+SA)

The antenna system provides M2^(N) beams circularly placed at angles of360°/M2^(N) one to each other. The beams will be divided into M groups:G1, G2, . . . , GM, each having 2^(N) beams. If beams are numbered incircular order from 1 to M2^(N), then G1 will contain beams B11=1,B12=M+1, B13=2M+1, . . . , while G2 will contain beams B21=2, B22=M+2,B23=2M+2, . . . , etc. Each group of antennas will operate on the samefrequency and different groups will operate on different frequencies.

Referring to FIG. 5 a and 5 b there are illustration examples ofgrouping for M2^(N)=16. Note that M=8, N=1 and M=16, N=0 are alsopossible solutions. FIG. 5 a shows M=4, N=2 and FIG. 5 b shows M=2, N=3.Each group of beams is processed by one distribution switch 54 thatallows 1, 2, . . . , or 2^(N) transceivers 52 to cover allsubscriber-stations in all 2^(N) beams. This is achieved usingtime-division-multiple-access (TDMA).

Referring to FIG. 6, there is illustrated in further detail thedistribution switch of FIG. 4. The four way distribution switch 54includes a plurality of inputs 60 a-60 d for coupling to correspondingtransmitters T1-T4 and a plurality of outputs 62 a-62 d for coupling tocorresponding beams B1-B4. The four way distribution switch 54 includesfirst and second cross connect switches 64 and 66 coupled in seriesbetween inputs 60 a and 60 b and outputs 62 a and 62 b. A third crossconnect switch 68 coupled to outputs 62 c and 62 d having a first inputcoupled to a second output of cross connect switch 64. The cross connectswitch 64 also includes straight switches 70 and 72. Straight switch 70coupled to input 60 c and 72 coupled to input 60 d. Straight switch 70having an output coupled to a second input of cross switch. 66 andstraight switch 72 having an output coupled to a second input of crossswitch 68.

The distribution switch is important because it connects one group of2^(N) beams to one transceiver or 2 transceivers or so on up to 2^(N)transceivers. To understand its operation we use an example for N=2,then we show how it can be extend to N=3, 4, etc.

FIG. 6 shows the structure of the 4-way distribution switch (i.e. N=2).It connects 4 beams B1, B2, B3 and B4 to one, two, three or fourtransceivers: T1, T2, T3, T4. The distribution switch is built with 3cross-switches: XS20, XS10 and XS11, and two straight switches SS21a andSS21 b.

The cross switches can be configured in four modes:

-   -   1. Straight: port A connects port C and port B connects port D,        both with 3 dB insertion loss    -   2. Cross: port A connects port D and port B connects port C,        both with 3 dB insertion loss    -   3. A-only: port A is split/combined to ports C and D    -   4. B-only: port B is split/combined to ports C and D

As described below, the cross-switch at IF or RF is implemented usingswitches and 3 dB splitters/combiners; thus, it introduces 3 dBinsertion loss plus losses due to imperfections. The straight-switchesmust introduce 3 dB insertion loss in order to balance the insertionloss of the cross-switches. The straight switches can be used tointroduce additional isolation when either T3 or T4 are not in use orthey can be simple 3 dB attenuators connecting port A with port B. Moredetails can be found below, where the construction of these switches isdescribed.

When deploying the system, the service provider will likely decide thata single transceiver is enough to cover all four beams. The transceiveris connected to T1 and the BS controller instructs the distributionswitch that T1 can manipulate all cross switches. Therefore, T1 coversall four beams: B1, B2, B3 and B4 using the following configurations:TABLE A One transceiver over 4 beams configurations Configuration XS20XS10 XS11 Mode Description Straight Straight — Tx or T1transmits/receives Rx through B1 Straight Cross — Tx or T1transmits/receives Rx through B2 Cross — Straight Tx or T1transmits/receives Rx through B3 Straight — Cross Tx or T1transmits/receives Rx through B4 A-only A-only A-only Tx T1 transmits onB1, B2, B3, B4 (downlink broadcasts) A-only A-only A-only Rx T1 receivesfrom B1, B2, B3, B4 (uplink contention windows) Straight A-only — Rx T1receives from B1, B2 (BSA - see below) Cross — A-only Rx T1 receivesfrom B3, B4 (BSA - see below)

When the service provider (SP) determines that the single transceiver 52a ₁ is overloaded, i.e. the data bandwidth provided by one transceiveris not enough, the SP can upgrade the system to two transceivers. Thesecond transceiver 52 a ₂ is added to port T2 without interfering withthe operation of the existing transceiver 52 a ₁. The BS controllerconfigures XS20 (64) as straight (A connects C and B connects D) andinstructs the distribution switch 54 a to allow T1 (60 a) to controlXS10 (66) and T2 (60 b) to control XS11 (68). Therefore, T1 (60 a)covers two beams: B1 and B2, and T2 (60 b) covers the other two beams:B3 and B4.

Depending on the service growth, the service provider may need tofurther upgrade the system. If T1 (60 a) is overloaded, a thirdtransceiver 52 a ₃ can be added at port T3 (60 c); the BS controllerconfigures XS10 (66) as straight and will leave T2 (60 b) to controlXS11 (68) (XS20(64) was already configured straight); T1(60 a) coversbeam B1, T3 (60 a) covers B2, and T2(60 b) covers B3 and B4. If T2(60 b)is overloaded, a transceiver can be added at port T4(60 d); the BScontroller configures XS11(68) as straight and leaves T1(60 a) tocontrol XS10(66); T1(60 a) covers B1 and B2, T2(60 b) covers B3, andT4(60 d) covers B4. Finally, if all four transceivers are used, the BScontroller configures all 3 cross switches (64,66,68) as straight anddoes not let any transceiver to control any cross switch. Then, T1(60 a)covers B1, T2(60 b) covers B3, T3(60 c) covers B2 and T4(60 d) coversB4.

The same paths used to upgrade to more transceivers can also be used todowngrade to fewer transceivers. The distribution switch 54 offers manyother configurations that can be used for making the system 50 failsafe.

Referring to FIG. 7 there is illustrated all useful configurations thatcan be obtained with the 4-way distribution switch. The five whiteblocks show the configurations discussed above, i.e. theupgrade-downgrade paths. The shaded configurations are not recommendedfor upgrade/downgrade; which provides the same functionality as thewhite, non-shaded configurations there is less upgrade/downgradeflexibility. However, shaded configurations can be used to provideback-off possibilities in the event that one or more transceivers fail.With two or more transceivers installed in the system, if any of thetransceivers fails, the distribution switch can always be reconfiguredsuch that the remaining transceivers cover all beams. When alltransceivers are installed, the system becomes immune to failure of anytwo transceivers.

Referring to FIG. 8 there is illustrated an 8-way distribution switch(N=3). The 8-way switch includes eight inputs 60 a, . . . 60 i fortransceivers T1, . . . T8 and eight outputs 62 a, . . . 62 i for beamsB1, . . . B8. Between inputs 60 a and 60 b and outputs 62 a and 62 b arethree cross switches 70, 72, and 74, each having first and second inputs(A, B) and first and second outputs (C, D) series connected at firstinputs/outputs to the output 62 a. A fourth cross switch 76 has itsfirst and second outputs coupled to outputs series connected to theinput 62 e and cross switch 80 has its second output coupled to theoutput 62 f. A seventh cross switch 82 has its first and second outputscoupled to outputs 62 g and 62 i, respectively. The input 60 h isconnected is connected to the second input (B) of the cross switch 70,whose second output (O) is connected to the first input (A) of crossswitch 78. The input 60 c is coupled via a straight switch 90 to thesecond input (B) of cross switch 72, whose second output (D) isconnected to the first output (A) of cross switch 76. The input 60 d iscoupled via a straight switch 92 to the second input (B) of cross switch78, whose second output (D) of cross switch 82. The input 60 e iscoupled via straight switches 94 and 96 to the second input (B) of crossswitch 74 whose second output (D) is connected to the output 62 b. Theinput 60 f is coupled via the straight switches 98 and 100 to the secondinput (B) of cross switch 76. The input 60 h is coupled via the straightswitches 102 and 104 to the second input (B) of cross switch 80. Theinput 60 i is coupled via the straight switches 106 and 108 to thesecond input (B) of cross switch 82.

The 8-way distribution switch is constructed with two 4-way distributionswitches, whose T1 ports are passed through the cross-switch XS30(70) toobtain the T1(60 a) and T2(60 b) ports for the 8-way distributionswitch. The other three T ports in each of the 4-way switches are passedthrough straight-switches to obtain the T3 . . . T8 ports for the 8-wayswitch. Using the same rule, two 8-way switches can construct a 16-waydistribution switch (N=4) and so on.

Referring to FIG. 9 there is illustrated the upgrade-downgrade paths forthe 8-way distribution switch of FIG. 8. The switch can connect anynumber of transceivers between 1 and 8 (60 a-60 i). The service providerhas the option of upgrading the system only when needed. If atransceiver is overloaded and covers two or more beams, its payload canalways be split with a newly added transceiver. Both the upgrades andthe downgrades do not require system shutdown and can be performedwithout any interruption of the ongoing communications.

When using more than one transceiver, if one transceiver fails, theswitch can be reconfigured such that all beams are covered.

Similarly a 2^(N)-way distribution switch can be built that allowstransceivers T1, T2 to cover 1, 2, 4, . . . , 2^(N) beams, transceiversT3, T4 to cover 1, 2, . . . , 2^(N-1) , T5, T6, T7, T8 to cover 1, 2, .. . , 2^(N-2) and so on. The fail-safe feature comes from the fact thatfor each sub-tree there are two transceivers that can cover the entiresub-tree.

Based on the structure of the switch, the number of beams that aparticular transceiver covers in any configuration is always a power of2. This helps with the development of the algorithms that will reside ineach transceiver and will ensure coverage of the required number ofbeams.

FIG. 10 shows a possible implementation of the cross-switch with two5-terminal dual-pole-dual-terminal (DPDT) RF/IF switches: DPDT1 andDPDT2, and two 3 dB splitters/combiners made by SC1, SC2 and twotermination impedances Z₀. The operation of the cross-switch isdescribed in Table. TABLE B Cross-switch operation Cross- DPDT1 DPDT2switch Connections Connections Made 1-4, 2-5 1-3, 2-4 Straight 1-3, 2-41-4, 2-5 Cross 1-4, 2-5 1-4, 2-5 A-only 1-3, 2-4 1-3, 2-4 B-only

Note that, if same power level P [dB] is applied to ports A and B, thenthe power delivered at ports C and D under all configurations is P−3 dB(minus some negligible loss due to circuit imperfections). Therefore,the distribution switch will deliver the same power to each active beam,which means that the antenna system will deliver constant EIRPregardless of configuration of the distribution switch.

Note that insertion loss in the receive direction from either C or D toeither A or B is constant (3 dB plus loss due imperfections) as long asthe path is active. This means that the receiver sensitivity is constantregardless of configuration of the distribution switch.

Depending on the performance of the straight-switches in terms ofinsertion-loss and isolation, the straight-switch can be:

-   -   1. a simple 3 dB attenuator (switch is always closed)    -   2. a 3 dB attenuator series with an single-pole-single-terminal        (SPST) RF/IF switch with no impedance matching    -   3. a 3 dB attenuator series with an SPST RF/IF switch with        impedance matching.

FIG. 11 shows a possible implementation of the straight-switch 120 as anSPST switch with impedance matching. The implementation uses a4-terminal DPDT RF/IF switch as switching element. With the DPDT switch,if terminal 1 is connected to 4, then the straight-switch is closed(ports A and B are connected); if terminal 1 connects to 3 and terminal2 to 4, then ports A and B are disconnected and each of them isterminated to ground with Z₀ (e.g. 50Ω). A 3 dB splitter/combiner isplaced in series with the DPDT switch. This can be replaced by a simple3 dB attenuator. To obtain an SPST switch without impedance matching,the two termination impedances Z₀ connected to the switch are removedfrom the circuit and the DPDT switch is replaced by a simple SPDT switch(placed between terminals 1 and 4).

Referring to FIG. 12 there is illustrated the protocol for one MAC framefor TDM/TDMA access to 2^(n) beams. In order to cover 2^(n) beams: B1,B2, . . . ,B2 ^(n), a transceiver T accesses the beams using acombination of time-division-multiplexing (TDM) andtime-division-multiple-access (TDMA). The following statements describethe operation with TDM/TDMA in detail. Both broadcast and unicast partsof the downlink are transmitted on all beams at the same time. Note thatthere is no overlap between beams and thus the beam gain and the beamshape are preserved on all beams. The information for different beams ismultiplexed in time using TDM.

On the uplink, during contention windows, T receives signals from allbeams. Again, since beams do not overlap, the beam gain and the beamshape are preserved on all beams. This permits new subscriber stations(SS) to register into the system and/or permits registered SSs torequest bandwidth (as provided by some standards).

Referring to FIG. 13, there is illustrated in a flow chart a beamselection in accordance with an embodiment of the present invention.After initial registration in the contention window, during thesubsequent n-frames, the SS will be polled n−1 times in thebeam-selection-algorithm (BSA) part of the uplink. The polling in BSA isused by the transceiver in the BS to discover the beam it shall use tocommunicate with the new SS. During the first polling the transceiver Tturns off 2^(n−1) beams and receives the combined signal from the other2^(n−1) beams. With either successful or unsuccessful reception, the BSwill know which group of 2^(n−1) beams the SS belongs. During the nextpolling the BS turns off 2^(n−2) of the 2^(n−1) beams and so on.

For all registered stations with known location (beam), the BS receivesthe uplink by steering the beam to desired direction. This to minimizethe interference at the receiver input. Thus, the information pertainingto different beams is multiplexed in a TDMA fashion on the uplink. Notethat it not necessary to group the uplink bursts by beam. The systemwill have the same performance if the uplink bursts are not grouped bybeam. The same applies to the downlink since the entire downlink isbroadcasted to all beams.

An alternate access method that does not require the use of BSA is shownin FIG. 14. The beams are multiplexed using TDM on the downlink and TDMAon the uplink, as in the previous solution. However, in order todiscover the beam for a new SS, the registration contention window isactive on single beam Bi at a time. Bi is changed every MAC frame suchthat all beams are covered in 2^(n) MAC frames. This method simplifiesthe control of the distribution switch but may introduce significantdelays during initial registration of a new SS if 2^(n) is large.

1. A method of beam steering in a wireless network comprising the stepsof: generating a first plurality of signals, each of the first pluralityof signals including a second plurality of signals, each signalcompatible with time division multiple access and time divisionmultiplexing; distributing the first plurality of signals to acorresponding first plurality of antennas; and passively forming asecond plurality of beams.
 2. A method as claimed in claim 1 whereineach first plurality is M, where M is an integer.
 3. A method as claimedin claim 2 wherein the second plurality is 2^(N), where N is an integer.4. A method of beam steering in a wireless network comprising the stepsof: generating a first plurality of signals, each of the first pluralityof signals including a second plurality of signals, each signalcompatible with time division multiple access and time divisionmultiplexing; distributing the first plurality of signals to acorresponding first plurality of antennas; and passively steering asecond plurality of beams.
 5. A method as claimed in claim 4 whereineach first plurality is M, where M is an integer.
 6. Apparatus for beamsteering in a wireless network comprising: means for generating a firstplurality of signals, each of the first plurality of signals including asecond plurality of signals, each signal compatible with time divisionmultiple access and time division multiplexing; means for distributingthe first plurality of signals to a corresponding first plurality ofantennas; and means for passively forming a second plurality of beams.7. Apparatus as claimed in claim 6 wherein each first plurality is M,where M is an integer.
 8. Apparatus as claimed in claim 7 wherein thesecond plurality is 2^(N), where N is an integer.
 9. Apparatus for beamsteering in a wireless network comprising: means for generating a firstplurality of signals, each of the first plurality of signals including asecond plurality of signals, each signal compatible with time divisionmultiple access; means for distributing the first plurality of signalsto a corresponding first plurality of antennas; and means for passivelysteering a second plurality of beams.
 10. Apparatus as claimed in claim9 wherein each first plurality is M, where M is an integer. 11.Apparatus as claimed in claim 10 wherein the second plurality is 2^(N),where N is an integer.